1. Technical Field
The present invention relates to a method of preparing a toner, and more particularly, to a method of preparing a toner, wherein a fixing property and storage stability at a high temperature are simultaneously improved as it is easy to control a structure of the toner.
2. Background Art
Recently, a need for a toner suitable for a high-speed printing, particularly, having a small volume average particle size and a narrow particle size distribution and provides an improved fixing property and storage stability at a high temperature, is increasing in the printing industry.
Generally, a toner is prepared by adding a colorant, a charge control agent, a releasing agent, or the like, to a thermoplastic resin acting as a binder resin. In addition, inorganic or metal fine particles such as silica or a titanium oxide may be added to a toner as external additives in order to provide a toner with fluidity or improve its physical properties such as charge controlling properties or cleaning properties.
The method of preparing such a toner can be categorized into physical and chemical methods.
The physical methods include a pulverization method, which has been widely used. The pulverization method is a method of preparing a toner by obtaining a toner composition by melt-mixing a colorant, charge control agent, and the like, with a binder resin, such as a polyester resin and dispersing the melt-mixture homogenously, and then, pulverizing and classifying the toner composition. The types of resin used in the pulverization method are not limited, but since particles are prepared by pulverizing the resin with a shearing force, huge energy is used, and it is difficult to reduce the sizes of the particles. Also, a particle size distribution is wide since the sizes of the particles are not uniform, and thus, separating out particles having large diameters and small diameters outside a predetermined range should be added so as to improve the resolution of a printed image. Also, when an additive is not uniformly dispersed, the fluidity, developing property, durability, and image quality of the toner may deteriorate. Accordingly, the pulverization method is not suitable for preparing a toner for a high-speed printer.
Meanwhile, the chemical methods include a suspension-polymerization method and an emulsion-aggregation method.
The suspension-polymerization method is disclosed in U.S. Pat. No. 6,177,223, and is a method of preparing a toner by suspension-polymerizing toner materials including binder resin monomers. Particles prepared by using the suspension-polymerization method are spherical in shape, the sizes of the particles can be reduced, and huge energy is not required. However, in the suspension-polymerization method, the types of resin are limited to a vinyl resin that is capable of polymerization in the presence of a radical initiator. Also, polymerization is not completely performed in the suspension-polymerization method. Accordingly, a untreated monomer or solvent may be left in the prepared particles, and thus, penetrate through the surfaces of the particles, thereby deteriorating the physical properties of toner particles and emitting lots of volatile organic compounds (VOC).
The emulsion-aggregation method is disclosed in U.S. Pat. Nos. 5,916,725 and 6,268,103, and is a method of preparing a toner by preparing a micro-emulsion resin particle composition through an emulsion polymerization reaction, and then aggregating the composition with other toner components, e.g. a pigment dispersion and a releasing agent dispersion. According to the emulsion-aggregation method, the shapes of toner particles may vary by adjusting aggregating conditions; for example, the toner particles may have non-spherical shapes. However, since the emulsion-aggregation method also uses emulsion polymerization, only vinyl resins, such as styrene-acrylic copolymers, may be used as binder resins, and preparing of a separate dispersion solution should be further included.
According to such chemical methods, polymerization is performed in a solution, and thus, only vinyl resins may be used as binder resins. However, it is preferable to use polyester resins instead of vinyl resins for a toner for a high-speed printer. That is, the polyester resins are suitable for a high-speed printer or a color printer since the polyester resins have improved pigment dispersibility, excellent transparency, low fixing temperature, and a narrow glass transition temperature range as compared to styrene-acrylic copolymers.
Consequently, a chemical method of preparing a toner using polyester resins has been investigated a lot recently.
For example, JP Patent Nos. 3,640,918 and 3,878,537 disclose a method of preparing toner particles by dissolving toner components in an organic solvent, and then, dispersing the resultant in an aqueous phase including surfactant. According to such a method, the types of a binder resin may be selected freely and the amount of VOC may decrease, but the toner components should be pre-added to the organic solvent, and thus, it is difficult to control a structure of the prepared toner particles.
Accordingly, a method of preparing a toner, wherein a small volume average particle size, a narrow particle size distribution, an improved fixing property, and an improved storage stability at a high temperature are obtained by controlling a structure of a toner, is still required to be developed.